The present invention relates generally to material specimen inspection techniques, and, more particularly, to a system and method for improving spatial resolution of electron holography.
Advancements in Transmission Electron Microscopy (TEM) technology enable materials to be analyzed at near atomic resolution by providing high-magnification, high-resolution imaging and analysis capabilities. TEM enables scientists to gather information relating to a material's physical properties, such as its microstructure, crystalline orientation and elemental composition. This information has become increasingly important as the need for advanced materials for use in areas such as microelectronics and optoelectronics, biomedical technology, aerospace, transportation systems and alternative energy sources, among others, increases.
TEM is accomplished by examining material specimens under a transmission electron microscope. In a transmission electron microscope, a series of electromagnetic lenses direct and focus an accelerated beam of electrons, emitted from an electron gun contained within the microscope, at the surface of a specimen. Electrons transmitted through the specimen yield an image of the specimen's structure, which provides information regarding its properties. In addition, elemental and chemical information is provided by both the transmitted electrons and the x-rays that are emitted from the specimen's surface as a result of electron interaction with the specimen.
In 1947, a Hungarian-British physicist named Dennis Gabor sought to find a way to sharpen the resolution of the images initially produced in transmission electron microscopes, which were in their infancy at the time. He proposed electron holography, a method of interference imaging in which the phase and amplitude components of the electron beam are separated to correct the spherical aberration of the microscope. In this regard, the electron beam source is split into the incident, undeviated electron wave (i.e., the reference wave) and the image wave (or object wave) diffracted by the specimen and exiting the bottom surface thereof. Assuming the electron optical geometry is correctly set up, these two waves can be made to interfere. The resulting interference pattern is then processed using optical techniques to form the holograms (images).
Unfortunately, the electron microscopes of Gabor's era did not produce an electron wave with sufficient coherence to permit the proper degree of interference required to make a useful hologram. More recently however, the development of TEMs using highly coherent field-emission electron sources has made electron holography a more effective undertaking. This technique has been shown to be particularly valuable for two-dimensional, p-n junction potential mapping of semiconductor devices with high spatial resolution. Such information is valuable for semiconductor device development and yield improvement.
Recent scaling in semiconductor device manufacturing down to sub-micrometer levels (e.g., about 20–100 nm gate length devices) warrants the implementation of two-dimensional electrical junction mapping at a high spatial resolution (e.g., less than 1 nm). In this regard, off-axis electron holography in a TEM has been successfully used to produce 2-D potential maps of semiconductor devices at a spatial resolution from which junction positions can be inferred. The potential maps are derived from phase information that is extracted from the interference pattern generated by overlapping electrons, which have passed through the semiconductor sample along with the electrons that have not.
As devices have continued to shrink, the corresponding electron holography parameters for effective inspection of the current generation of these semiconductor devices now include: an overlap width in the range of about 100 nm to about 1000 nm for an adequate field of view (FOV); a fringe spacing between about 0.1 nm to about 10 nm for meaningful spatial resolution; visibility of the fringe contrast of about 10% to about 30% for useful signal to noise ratio; and adjustability of both the overlap and the fringe spacing relative to the sample for flexibility. FIG. 1(a) is an electron hologram taken with no sample, illustrating the fringe width (which determines the FOV), while FIG. 1(b) is an enlarged view of the highlighted portion of FIG. 1(a), particularly illustrating the fringe spacing (which determines the spatial resolution).
The implementation of previous approaches to off-axis electron holography (i.e., utilizing a single objective lens) has resulted in a trade off between fringe width (FOV) and fringe spacing. For example, if the overlap width was satisfactory, the fringe spacing was unsatisfactory and vice versa. Accordingly, it has now desirable to be able to independently control the various electron holography parameters, such as overlap and fringe spacing, for adequate inspection of increasingly scaled down semiconductor devices.